1. Field of the Invention
The present invention generally relates to filtering circuits comprised of a parallel resonant circuit, and more particularly, to parallel resonant circuits that directly connect to AC power sources without any intervening elements, reducing all frequency distortions in alternative currents, including harmonic distortion.
2. Description of Related Art
In general, as illustrated in the prior art FIG. 1A, parallel resonant circuits 6 comprised of a capacitor 12 connected in parallel to an inductor 10 are always connected to a power source 2 through one or more electrical components 8. Each of the components 8A and 8B may for example comprise of one or more inductors to isolate a load 4 from a source 2, one or more resistors to dampen oscillations or dissipate power, or some other elements to perform other functions. The components 8 do not represent inherent or intrinsic characteristics of any electrical component, but represent extrinsic, additional components such as actual resistors or inductors. The circuit topography comprised of the parallel resonant circuits 6 coupled with at least one or more other components 8 is purported to reduce harmonic distortions in an alternative current waveform, in addition to the functions described above, with the additional functions depending on the type(s) of element(s) 8 always connected to the parallel resonant circuit 6.
When AC current flows through the inductance 10 a back electromotive force (emf) or voltage develops across it, opposing any change in the initial AC current. This opposition or impedance to change in current flow is measured in terms of inductive reactance. The inductive reactance is determined by the formula:ZL=(2πfL)  (1)Where
f=Operating Frequency
L=Inductance
ZL=Reactive Impedance of the Inductor.
When AC voltage develops across the capacitor 12, an opposing change in the initial voltage occurs, this opposition or impedance to a change in voltage is measured in terms of capacitive reactance. The capacitive reactance is determined by the formula:Zc=1/(2πfC)  (2)Where
f=Operating Frequency
C=Capacitance
Zc=Reactive Impedance of the Capacitor.
Resonance for circuit 6 occurs when the reactance ZL of the inductor 10 balances the reactance ZC of the capacitor 12 at some given frequency f. The resonance frequency is therefore determined by setting the two reactance equal to one another and solving for the frequency, f.(2πfL)=1/(290 fC)  (3)This leads to:fRE=1/2π√LC  (4)Where
fRE=Resonant Frequency.
In general, the parallel resonant circuits present very high impedance to those electrical signals that also operate at the same resonant frequency, fRE. At resonance, input signals with frequencies becoming far removed from the resonance frequency fRE see ever-decreasing impedance presented by the parallel resonant circuit. For example, if parallel resonant circuit 6 illustrated in FIG. 1A is tuned to resonate at the fundamental frequency of the power source 2, where fRE=fFUND, the input current signals from power source 2 that operate at frequencies equal to fFUND will be rejected by circuit 6 and will pass onto the load 4. To these current signals, the parallel resonant circuit 6 is almost invisible because it behaves almost like an “open circuit” at fRE=fFUND. As the input current signals depart from the resonant frequency, up or down, the parallel circuit 6 presents a lessening impedance and progressively allows other signals (those not operating at fRE) to leak to ground. For signals at frequencies far removed from resonance, the parallel resonant circuit 6 presents a short path to ground. Using these principles, parallel resonant circuits 6 may be tuned to the fundamental frequencies of the power source 2 to therefore filter out frequencies above or below the fundamental, providing low noise signals to load 4. The filtering action is mainly done by the capacitance portion of the parallel resonant circuit, with the inductance part “giving back” the capacitive current drawn by the capacitor. In general, one may look at the impedance presented by the parallel resonant circuit in terms of its capacitive impedance ZC of equation (2) above. Accordingly, for high frequencies the denominator of equation (2) having the frequency value f will increase, making the total impedance of the parallel resonant circuit smaller.
The amount of noise on signals passed on to load 4 depend mostly on how much of lessening impedance any path to ground presents for input signal with operating frequency above or below the desired operating frequency. In particular, the total impedance of any path to ground must be considered to determine the appropriate filtering effect for signals with undesirable frequencies, and not just that of the parallel resonant circuit. In the instance of FIG. 1A, the total impedance includes that presented by the parallel resonant circuit 6 and those of any component 8 coupled thereto. Therefore, the total impedance of a path to ground for signals with undesirable operating frequency will not behave as a shorted path even if the parallel resonant circuit behaves ideally and presents a “short circuit” behavior. Components 8 will still maintain and present impedance commensurate with their rated values, regardless of any frequency variations. Accordingly, the true impedance of the circuit path to ground for the combination of the parallel resonant circuit 6 and the components 8 is given by:ZTOTAL=ZPRC+Z8  (5)Where    Z8=Impedance of elements 8A or 8B.    ZPRC=Impedance of the Parallel Resonant Circuit    ZTOTAL=Total impedance.
FIG. 1B graphically illustrates the consequence of the additional impedance Z8 of component(s) 8. As shown, as the frequency f increases (moves away from the resonant frequency), the total impedance ZTOTAL illustrated by line 14 decreases, allowing short path for current signals with undesirable frequencies to ground, filtering these signals. However, even if the frequencies become very large where the ZPRC of the ZTOTAL becomes almost zero, ZTOTAL itself will than equal to Z8. Hence, for frequencies much higher than those desired, equation (5) will equal:ZTOTAL=0+Z8  (6)
ZTOTAL can never present a short circuit path for signals with frequencies removed from the desired operating frequency due to impedance of one or both of the elements 8A and 8B. Hence, all the undesirable frequencies illustrated in region 16 of the graph will continue to be passed on to the load 4, regardless of how low of an impedance the parallel resonant circuit 6 presents to the signals that operate away from the resonant frequency.
As a specific example, U.S. Pat. Nos. 5,323,304 and 5,570,006, both to Woodworth, the entire disclosures of which are incorporated herein by reference, teach in their respective FIG. 1 the use of parallel resonant circuit 20 coupled through an inductor 21 to a power source 12. In this instance, the inductor 21 would constitute the elements 8A of the prior art FIG. 1A of the present invention. As taught in Woodworth, the series connected inductor 21 isolates the power source 12 from the load 16 such that harmonic currents that may be generated by the load 16 will minimally affect the power source 12. In addition, the inductor 21 also serves to increase the effective impedance of the power source 12 as seen by the load 16, limiting the amount of power that can be drawn by the load. This increase in effective impedance (Z8 of the inductor 21) degrades the filtering effect of the parallel resonant circuit, and as illustrated in prior art FIG. 2A, distorts the output current and voltage supplied to a load.
U.S. Pat. No. 3,237,089 to Dubin et al shows a similar circuit where inductor Ls is connected in series with the parallel resonant circuit LC, comprised of an inductor L connected in parallel with a capacitor C. The circuit topography illustrated is a simplified equivalent circuit of a saturable-type constant voltage transformers, where inductor Ls isolates the power source ei from a load. This circuit is illustrated only for as a way to show how a constant voltage transformer functions. Therefore, the reference U.S. Pat. No. 3,237,089 is only concerned with voltage level control, and not filtering action.
Many electronic devices (loads) today draw current only at the peaks of the sinusoidal AC power supply voltage. This cause the peaks of the AC supply waveform to become flattened out because of this non-linear loading of the power grid, reducing the amount of power supply required by loads. As illustrated in the prior art FIG. 2A, this is easily detected by measuring the amount of current IL 20 drawn by load 4 of FIG. 2B, and the sinusoidal voltage VL 18 across the load 4. The current drawn by the load 4 at the peak of the sinusoidal voltage causes the voltage waveform 18 to be flattened at its sinusoidal peak. The more loads are connected to a power source, the flatter the waveform of the voltage across those loads.
Adding components 8 (FIG. 1A) exasperate the above-described situation, worsening the flattening of the voltage waveform at the load. For example, the sudden draw of current 20 by load 4 at the peak of the voltage 18 produces an opposing voltage across inductor 24 (due to the inductive reactance), lowering even further the peak of the voltage 18 available to the load 4. Addition of components 8 distorts the voltage waveform 18 across the load 4, generating noise thereat. Noise is generated because load requirements for appropriate load current and voltage are not met. Hence, even low value inductors 24 in series with the power source 2 and the parallel resonant circuit 6 cause much trouble.
As another specific example, the U.S. Pat. No. 5,343,381 to Bolduc et al, the entire disclosure of which is incorporated herein by reference, teach in their FIG. 1 the use of a resistor element 8 connected in series with a parallel resonant circuit that is comprised of a capacitor 4 connected in parallel with an inductor 6 to produce a dampening circuit 2. In this instance, the resistor 8 of Bolduc et al would constitute the elements 8B of the prior art FIG. 1A of the present invention. The dampening resistor 8 degrades correction of any possible output distortions illustrated in prior art FIG. 2A of the present invention. In addition, the LC filtering effect is also degraded due to the added impedance of resistor 8, as graphically illustrated in the prior art FIG. 1B of the present invention. In this instance, the impedance Z8 illustrated in FIG. 1B equal the value of resistor 8.
As described and illustrated, parallel resonant circuits have always been connected to a power source through some other component that degrades or negates the resonant circuit's performance in terms of output signal correction and filtering of signals that operate at undesired frequencies.